Re: [PATCH RFC net-next 00/10] MC Flood disable and snooping

[Vladimir Oltean]

On Tue, Apr 02, 2024 at 09:50:51PM +0300, Nikolay Aleksandrov wrote:
> On 4/2/24 20:43, Vladimir Oltean wrote:
> > Hi Nikolai,
> > 
> > On Tue, Apr 02, 2024 at 12:28:38PM +0300, Nikolay Aleksandrov wrote:
> > > For the bridge patches:
> > > Nacked-by: Nikolay Aleksandrov <razor@xxxxxxxxxxxxx>
> > > 
> > > You cannot break the multicast flood flag to add support for a custom
> > > use-case. This is unacceptable. The current bridge behaviour is correct
> > > your patch 02 doesn't fix anything, you should configure the bridge
> > > properly to avoid all those problems, not break protocols.
> > > 
> > > Your special use case can easily be solved by a user-space helper or
> > > eBPF and nftables. You can set the mcast flood flag and bypass the
> > > bridge for these packets. I basically said the same in 2021, if this is
> > > going to be in the bridge it should be hidden behind an option that is
> > > default off. But in my opinion adding an option to solve such special
> > > cases is undesirable, they can be easily solved with what's currently
> > > available.
> > 
> > I appreciate your time is limited, but could you please translate your
> > suggestion, and detail your proposed alternative a bit, for those of us
> > who are not very familiar with IP multicast snooping?
> > 
> 
> My suggestion is not related to snooping really, but to the goal of
> patches 01-03. The bridge patches in this set are trying to forward
> traffic that is not supposed to be forwarded with the proposed
> configuration,

Correct up to a point. Reinterpreting the given user space configuration
and trying to make it do something else seems like a mistake, but in
principle one could also look at alternative bridge configurations like
the one I described here:
https://lore.kernel.org/netdev/20240402180805.yhhwj2f52sdc4dl2@skbuf/

> so that can be done by a user-space helper that installs
> rules to bypass the bridge specifically for those packets while
> monitoring the bridge state to implement a policy and manage these rules
> in order to keep snooping working.
> 
> > Bypass the bridge for which packets? General IGMP/MLD queries? Wouldn't
> > that break snooping? And then do what with the packets, forward them in
> > another software layer than the bridge?
> > 
> 
> The ones that are not supposed to be forwarded in the proposed config
> and are needed for this use case (control traffic and link-local). Obviously
> to have proper snooping you'd need to manage these bypass
> rules and use them only while needed.

I think Joseph will end up in a situation where he needs IGMP control
messages both in the bridge data path and outside of it :)

Also, your proposal eliminates the possibility of cooperating with a
hardware accelerator which can forward the IGMP messages where they need
to go.

As far as I understand, I don't think Joseph has a very "special" use case.
Disabling flooding of unregistered multicast in the data plane sounds
reasonable. There seems to be a gap in the bridge API, in that this
operation also affects the control plane, which he is trying to fix with
this "force flooding", because of insufficiently fine grained control.

> > I also don't quite understand the suggestion of turning on mcast flooding:
> > isn't Joseph saying that he wants it off for the unregistered multicast
> > data traffic?
> 
> Ah my bad, I meant to turn off flooding and bypass the bridge for those
> packets and ports while necessary, under necessary can be any policy
> that the user-space helper wants to implement.
> 
> In any case, if this is going to be yet another kernel solution then it
> must be a new option that is default off, and doesn't break current mcast
> flood flag behaviour.

Yeah, maybe something like this, simple and with clear offload
semantics, as seen in existing hardware (not Marvell though):

mcast_flood == off:
- mcast_ipv4_ctrl_flood: don't care (maybe can force to "off")
- mcast_ipv4_data_flood: don't care
- mcast_ipv6_ctrl_flood: don't care
- mcast_ipv6_data_flood: don't care
- mcast_l2_flood: don't care
mcast_flood == on:
- Flood 224.0.0.x according to mcast_ipv4_ctrl_flood
- Flood all other IPv4 multicast according to mcast_ipv4_data_flood
- Flood ff02::/16 according to mcast_ipv6_ctrl_flood
- Flood all other IPv6 multicast according to mcast_ipv6_data_flood
- Flood L2 according to mcast_l2_flood